Expert Committee Recommendations for using A1c for the Diagnosis of Diabetes: Diabetes Care, August 2009
There is no single assay related to hyperglycemia that can be considered the gold standard, as it relates to the risk for microvascular or macrovascular complications.
A measure that captures chronic glucose exposure is more likely to be informative regarding the presence of diabetes than is a single measure of glucose.
The A1c assay provides a reliable measure of chronic glycemia and correlates well with the risk of long-term diabetes complications.
The A1c assay (standardized and aligned with the Diabetes Control and Complications Trial/UK Prospective Diabetes Study assay) has several technical, including preanalytic and analytic, advantages over the currently used laboratory measurements of glucose.
For the reasons above, the A1c assay may be a better means of diagnosing diabetes than measures of glucose levels.
The diagnosis of diabetes is made if the A1c level is >=6.5%. Diagnosis should be confirmed with a repeat A1c test unless clinical symptoms and glucose levels >200 mg/dl (>11.1 mmol/l) are present.
If A1c testing is not possible owing to patient factors that preclude its interpretation (e.g., hemoglobinopathy or abnormal erythrocyte turnover) or to unavailability of the assay, previously recommended diagnostic measures (e.g., FPG and 2HPG) and criteria should be used. Mixing different methods to diagnose diabetes should be avoided.
In children and adolescents, A1c testing is indicated when diabetes is suspected in the absence of the classical symptoms or a plasma glucose concentration >200 mg/dl (>11.1 mmol/l).
The diagnosis of diabetes during pregnancy, when changes in red cell turnover make the A1c assay problematic, will continue to require glucose measurements.
For the Identification of Individuals at High Risk for Diabetes
Individuals with an A1c level >=6% but <6.5% are likely at the highest risk for progression to diabetes, but this range should not be considered an absolute threshold at which preventative measures are initiated.
The classification of subdiabetic hyperglycemia as pre-diabetes is problematic because it suggests that all individuals so classified will develop diabetes and that individuals who do not meet these glycemia-driven criteria (regardless of other risk factor values) are unlikely to develop diabetes -- neither of which is the case. Moreover, the categorical classification of individuals as high risk (e.g., IFG or IGT) or low risk, based on any measure of glycemia, is less than ideal because the risk for progression to diabetes appears to be a continuum. The glucose-related terms describing subdiabetic hyperglycemia will be phased out of use as clinical diagnostic states as A1c measurements replace glucose measurements for the diagnosis of diabetes.
When assessing risk, implementing prevention strategies, or initiating a population-based prevention program, other diabetes risk factors should be taken into account. In addition, the A1c level at which to begin preventative measures should reflect the resources available, the size of the population affected, and the anticipated degree of success of the intervention. Further analyses of cost-benefit should guide the selection of high-risk groups targeted for intervention within specific populations.